How to Tweak Dragon Scales Scaling from a Particle Emitter
Define a vertex group on your particle emitter, the tail, and use weight paint to make the part with larger scales stronger (red) and smaller scales weaker (blue). In your particle settings under "Vertex Groups" there is a field called "Length". Add the vertex group you just created to that. Now, the scales should be larger where the weight is high and shorter where the weight is low.

Hope that helps and answers your question.